There has been known an image forming apparatus including a plurality of photosensitive drums, a conveyance belt (a transfer member) facing the respective photosensitive drums and configured to convey a sheet, a plurality of transfer rollers configured to transfer developer images on the respective photosensitive drums to the sheet on the conveyance belt, a sensor configured to detect a test pattern transferred from the photosensitive drums onto the conveyance belt, and a fixing device arranged downstream of the conveyance belt in a conveyance direction (refer to JP-A-2008-52215). In this image forming apparatus, in order to suppress sensitivity of the sensor from being lowered due to heat applied to the sensor from the fixing device, components (a heating roller, a pressing roller and the like) in the fixing device are surrounded by a housing, so that the heat in the fixing device is suppressed from being transferred to the sensor.
In order to make the fixing device smaller and lightweight, the pressing roller may be configured to be exposed from the housing or the housing of the fixing device may be formed with a hole (a hole other than an opening through which the sheet passes). In this case, the heat in the fixing device may flow towards the sensor from a passage between the pressing roller and the housing or from the hole formed at the housing, so that the sensitivity of the sensor may be deteriorated. When the sensitivity of the sensor is deteriorated, a test pattern cannot be favorably detected, so that it is difficult to from a high-quality image.